By Toby Poston
Business reporter, BBC News
Colorado, America's "Centennial State", is home to four-and-a-half million people, lots of cattle, and potentially the world's biggest reserves of oil.
US senators have been visited oil shale sites in Colorado
The US Energy department thinks that the state is sitting on about a trillion barrels worth of oil, as much as the rest of the world's conventional oil reserves added together.
Trouble is, it's not quite ready to be extracted yet.
The state's "black gold" is trapped thousands of feet underground in the kerogen-rich shale rock deposits of western Colorado.
If it was allowed to sit tight, a few more million years of heat and pressure would transform it into liquid pools ready to be drilled.
Obviously, impatient prospectors are not prepared to wait, so they have made several attempts to tap this potentially huge resource, in the 1920s, 50s and 80s.
The most common technique used over the years has involved mining the rock, crushing it and then heating it to release the oil.
One scheme tried using steam injection, another suggested detonating a 50-kiloton atomic "device" underground.
But all attempts either failed at the research stage or were abandoned because the costs of producing the oil were uneconomic.
Many still recall "Black Sunday", 2 May 1982, when the energy giant Exxon closed its $5bn Colony Shale Oil Project and laid off more than 2,000 workers.
But now, with oil prices hovering around $70 per barrel, instability surrounding many of the world's biggest producers and demand surging across the globe, US politicians and energy companies are beginning to get excited again about Colorado's "rock that burns".
Last year's National Energy Policy Act asked the US Bureau of Land Management to issue research and development leases in the Green River Formation, an oil-shale rich region that stretches across western Colorado, eastern Utah and south-western Wyoming.
Shell is ten years into a long-term oil shale research project
Earlier this month, the Senate Energy and Natural Resources Committee held a field hearing in Colorado to get a closer look at the region's oil producing potential.
"We are more dependent on foreign oil than ever; our world is a more fragile and unstable place. Energy prices have soared," said the committee's chairman Pete Domenici.
"I believe that with private citizens, state and local governments and industry working together as a group we can make oil shale recovery work for America."
One of the big attractions for energy companies is that oil shale presents a much lower risk than many other types of oil exploration.
"It is not like spending hundreds of thousands of dollars drilling a hole that is dry," says Kyle Cooper, an oil analyst with IAF Advisors in Houston.
"They will find oil - it is just a question of whether or not it is economic.
"They don't want to spend billions building all the necessary infrastructure and then find that the oil price has dropped."
Cooking for oil
Energy group Royal Dutch/Shell has been conducting field research in Colorado's Rio Blanco County for 10 years.
It is drilling holes and inserting electric heaters to gradually heat the rock to a temperature of 650-700 degrees Fahrenheit over a number of years.
The resulting product is one third natural gas and two thirds light oil.
Surface processing can then turn the oil into products such as diesel, jet fuel or petrol.
Royal Dutch/Shell says it will decide on whether to start a commercial project by the end of the decade.
Large scale production in the region will depend upon whether companies like Shell think they can produce oil at an economically viable cost.
Shell is already extracting fuel from oil sands in Canada
But there is also the issue of the environmental consequences of oil shale exploitation.
A study commissioned by the US Department of Energy in 2005 pointed out that such production could cause huge land disruption and air pollution, as well as drawing a large number of people to a rural area.
The oil extraction process would be very energy and water intensive, requiring the building of new power stations and putting pressure on the region's already limited water supplies.
But with some reports claiming that America's oil shale deposits could deliver 100-years of energy independence, it is perhaps a question of when, rather than if, large scale production begins.